290 



SUPPLEMENT TO THE 



Having once got a lens whose focus we know for certain, 

 that of all others may be easily obtained by converting 

 them into the object-glasses of an engiscope, having a 

 micrometer in its eye-piece, and measuring with them the 

 size of the image of a division on another micrometer 

 placed on the stage, which, if twice as large as that given 

 by the inch lens, denotes a focus twice as long ; if only 

 half the size, one of half the length; if one-tenth, one- 

 tenth of the length, &c. 



Great care must be taken in these measurements to pre- 

 serve the same length of body exactly with all the different 

 lenses whose foci are to be measured ; and the apertures 

 must always be reckoned from the side of the lens next 

 the eye-piece, taking care that it is not altered by the effect 

 of stops in the body. 



The following table serves to shew the angle of aperture 

 of metals and object-glasses whose apertures bear the 

 following proportions to their foci: the measurements were 

 executed with a metal, in order that the thickness of glass 

 lenses might not operate to occasion uncertainty and 

 variation. 



Aperture expressed in Terms of the 

 Focus for parallel Rays. 



A 

 4 



1 



Angle of Aperture. 



Degrees. 



6* 

 13! 

 18J 



55 



